A radial gap type electric motor is used as a general electric motor of a drive source or the like of an electric two-wheeled vehicle or the like. The radial gap type electric motor is provided with a magnet around an axis in a cylindrical shape on a side of, for example, a rotor, and provided with a plurality of teeth opposed to a cylindrical face of the magnet on a side of a stator and wound with coils around the teeth. Therefore, a gap between opposed faces of the magnet and the respective teeth is formed in a cylindrical shape along the axis.
Meanwhile, an axial gap type electric motor is used as a rotation drive source of an audio apparatus or the like having a comparatively small output. The axial gap type electric motor is constituted by a yoke on a side of a rotor in a circular plate shape fixed to a rotating shaft, a yoke on a side of a stator in a circular plate shape opposed to the yoke on the side of the rotor, a magnet fixed to a side of an opposed face of the yoke of either one yoke on the side of the rotor or the side of the stator, a plurality of teeth arranged on the side of an opposed face of other yoke on the side of the rotor or the side of the stator radially and opposedly to the magnet and coils wound around respective teeth. Therefore, a gap between the opposed faces of the magnet and the teeth is formed in a planar shape orthogonal to an axis.
FIG. 17 is an explanatory view of a magnetic flux of an axial gap type electric motor of a background art. The drawing shows a magnetic flux only with respect to one tooth 3 and illustration thereof is omitted with respect to left and right contiguous teeth 3.
The stator 1 is provided with a stator yoke 2 in a circular plate shape having a laminated member of steel plates and a plurality of teeth 3 each similarly having a laminated member of steel plates which are arranged radially above the stator yoke 2. Each tooth 3 is wound with a coil (not illustrated). A rotor (not illustrated) in a circular plate shape is arranged opposedly to the teeth 3 of the stator. A magnet is fixed to the rotor at a predetermined gap from upper faces of the teeth 3. Incidentally, the circular plate shape includes a circular shape and a planar ring shape (doughnut shape).
A magnetic circuit is formed between the rotor, not illustrated, and the stator, and a magnetic flux coming out from an N pole of the magnet is made to flow to the tooth 3 and to the stator yoke 2 (arrow A) and flow to an S pole (not illustrated) of the magnet by passing other teeth 3. By energizing the coil, the tooth of that coil is excited to attract and repulse the magnet of the rotor opposed to an upper face of the tooth. By successively switching energization of the coil, the excited teeth are successively moved and the rotor is rotated along with the magnet.
According to such an axial gap type motor, opposed faces of the magnet and the teeth are orthogonal to an axial direction and therefore, a length in the axial direction becomes shorter than that of the radial gap type. Also in the case of increasing an output, the opposed faces opposed to each other via the gap can be increased without prolonging the length in the axial direction and therefore, the constitution can contribute to thin formation of the motor.
However, according to the above-described axial gap type electric motor, by energizing the coil, the magnetic flux flowing from the tooth 3 to the stator yoke 2 is changed in a direction and a magnitude thereof since the magnet on the side of the rotor is rotated and by electromagnetic induction in accordance with an amount of the change, an induced current B in an eddy shape is made to flow at inside of the stator yoke 2 centering on the tooth 3 at a surrounding thereof (FIG. 17). The induced current B becomes Joule's heat to constitute loss of energy and the motor efficiency is reduced.
Although the loss of energy by the heat does not cause a serious problem in the case of a low output, when a strong magnet is used for achieving a large torque as in, for example, an electric two-wheeled vehicle, the loss is significantly increased and also a temperature rise rate is increased to bring about high temperatures.
Therefore, although such an axial gap type electric motor is of a thin type and regarded to be preferable to mount to an axle or the like of an electric two-wheeled vehicle, the motor efficiency is significantly reduced in the case of the electric two-wheeled vehicle having a high torque and using a strong magnet and therefore, the axial gap type electric motor is not applied as the drive source.
The invention takes a consideration of the above-described background art and it is an object thereof to provide an axial gap type electric motor which is small-sized and achieves a high motor efficiency as a drive source having a high torque using a strong magnet by reducing energy loss by an induced current.